The field of the present invention relates to integrated circuits. More particularly, the field of the present invention relates to systems and methods for integrating electronic and optical circuits as a single integrated circuit.
Many telecommunications experts recognize that while present bandwidth needs are being met, satisfaction of those needs is not likely to continue indefinitely, without an advance in telecommunications technology. The typical consumer demands increasingly dense information content, and soon, perhaps even within the next few years, nothing short of fully interactive video, high-quality video on demand, and instant access to vast information resources, to name but a few types of high-bandwidth content, will be acceptable. Experts further recognize that a bandwidth bottleneck is likely to occur at the interface between the medium used for the backbone of global communications, i.e., fiber optics, and the media employed to complete the commonly termed “last mile” of communication, i.e., electronics-controlled wired and wireless-based systems, which connect the fiber-optic backbone to end-users such as businesses and households.
At this interface, the existing systems that manipulate (e.g., multiplex, demultiplex, transmit, receive, modulate, demodulate, and transduce components of) optical signals are far from seamless. These systems are costly, inflexible, bulky, employ many discrete components, are not as reliable as many would like, and most importantly, are unlikely to be able to meet the future bandwidth requirements.
One approach to improving these systems has been to develop optical amplifiers for use in different kinds of “optical integrated circuits.” Such optical amplifiers would be integrated with other passive optical devices, such as planar waveguides, splitters and multiplexers to form such optical integrated circuits. However, the development of optical amplifiers suitable for optical integrated circuits has been a significant technological hurdle.
For example, amplifying light in silica doped with an amplifying material, the most common medium for waveguide cores, is inherently limited to a gain of about 1 db per meter, which is insufficiently low for the small form factors necessary for an integrated optical circuit. The low available gain with pure fused silica is due to the low solubility of light amplifying materials, such as erbium and ytterbium, which could be doped into the silica. The light amplifying materials have optical transitions that enable signals at particular wavelengths to be amplified. Erbium, for example, has an optical transition that can be used successfully to amplify optical signals with wavelengths around 1550 nm. Using, for example, a 980 nm diode laser as a signal pump, a 1550 nm signal traveling through an erbium-doped waveguide induces stimulated emission resulting in signal amplification. Light amplifying materials such as erbium are critical to design of any optical amplifier in an optical integrated circuit. The inability to significantly incorporate such materials into silica makes it impractical as a waveguide medium for integrated optical circuits, which require especially high signal amplification due to their very short propagation lengths.
An effective alternative to silica has been found, however, in phosphate-based glasses. Phosphate-based glasses allow the incorporation of much more of the light amplifying materials than is possible in silica-based glasses. For example, “High Ultra-Short Length Phosphate Glass Erbium-Doped Fiber Amplifier Material,” by Lange et al., discloses the use of an erbium-doped phosphorous-based glass to amplify a signal in optical fiber over very short lengths. The article quotes a gain of 10 db in a 2.2 cm length of optical fiber. Such a gain is orders of magnitude greater than those possible in pure fused silica based glasses.
The higher available gain using phosphate-based glasses allows for the design of much smaller devices. For example, a 1×32 splitter can be manufactured in which each one of the 32 output channels to has the same power as the input signal in a length of a few centimeters. The same output per channel could be achieved in silica but the required length would be greater than 12 meters.
In addition to the benefit of higher available gain, phosphate-based glasses also combine such useful properties as lower processing temperature than silica based glasses, chemical durability, ion exchangeability, low upconversion losses and low concentration quenching. Phosphate based glasses suffer from relatively high signal attenuation, but in small-scale integrated optics devices powered by a pump laser, when the glass is combined with an light-amplifying material, such a limitation is not prohibitive.
Awareness of the utility of phosphate-based glass recently led to the demonstration of a practical waveguide amplifier suitable for the small form factors of an optical integrated circuit. In “Fiber-Device-Fiber Gain from a Sol-Gel Erbium-Doped Waveguide Amplifier,” Huang, et al. describe the preparation of a waveguide amplifier having a sol-gel core formed from aluminophosphosilicate glass doped with erbium and ytterbium. The article further discloses that the phosphate-based core requires a lower temperature for processing (i.e., rapid thermal annealing) than a pure silica-based glass.
The minimum required processing temperature for a waveguide becomes an issue if the processing of the waveguide core is in the presence of a substrate with an imprinted integrated circuit, which generally has a tolerance up to about 400° C. Epitaxial layers of gallium indium aluminum arsenide grown on gallium aluminum arsenide and other semiconductor materials (e.g., germanium, silicon) are desirable as substrates to foster integration of the optical elements with electronic elements on the same semiconductor. Pure fused silica-based glasses, however, generally have a minimum processing temperature of around 1200° C., making such glasses difficult to process while maintaining the integrity of the electronic integrated circuit. Phosphate-based glasses, on the other hand, can be processed at temperatures below 500° C., enabling such glasses to be processed on an electronic integrated circuit-imprinted semiconductor substrate.
Developments in integrated optics have made optical integrated circuits a reality. Methods for further integrating optics with electronics, however, have not been practically addressed.